Systems and methods for registering images obtained using various imaging modalities and verifying image registration

ABSTRACT

Embodiments of the present invention provide systems and methods to detect a moving anatomic feature during a treatment sequence based on a computed and/or a measured shortest distance between the anatomic feature and at least a portion of an imaging system.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 14/879,235, filed onOct. 9, 2015, the entire disclosure of which is hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to imaging, and, more specifically, tosystems and methods for registering images and/or verifying imageregistrations obtained using various imaging modalities.

BACKGROUND

Medical imaging of internal organs provides important anatomic anddiagnostic information, which medical personnel can employ to maketherapeutic decisions. Medical images can be acquired using variousnon-invasive imaging procedures, such as computed topography (CT),magnetic resonance imaging (MRI), or ultrasound imaging. A CT systemtransmits x-rays through an anatomic site of interest, and based on theattenuation coefficients of the x-rays, cross-sectional images(“slices”) of the object can be reconstructed. As a result, the CTsystem is well suited for viewing details about bony structures,diagnosing diseases of the lung and chest, and detecting cancers.Advantages of CT imaging also include, for example, a short scan timefor a complete scan (typically less than five minutes), low cost (abouthalf the price of an MRI apparatus), the ability to accurately outlinebony tissue inside the body, fewer motion artifacts due to fast imagingspeeds (each scan time is less than 30 seconds), etc. CT systems,however, irradiate the patient and pose consequent risk; for thisreason, CT scans are not recommended for pregnant women or children.

Ultrasound imaging, which involves passing high-frequency acoustic wavesthrough the body, is another widely used technique. Ultrasound wavespenetrate well through soft tissues and, due to their short wavelengths,can be focused to spots with dimensions of a few millimeters. In atypical ultrasound examination, a transducer probe is placed directly onthe skin or inside a body opening. A thin layer of gel may be applied tothe skin to provide direct contact between the transducer probe and skinand thereby allow efficient transfer of ultrasound energy into the body.An ultrasound image of internal anatomic structures can be constructedfrom the waves reflected by those structures—in particular, for example,from the amplitudes and phases of the reflection signals and/or the timeit takes for the ultrasound waves to travel through the body. Becauseultrasound images are captured in real-time, they can also show movementof the body's internal organs as well as blood flowing through the bloodvessels. In addition, ultrasound imaging offers high temporalresolution, high sensitivity to acoustic scatterers (such ascalcifications and gas bubbles), excellent visualization, low cost,portability, and no ionizing radiation exposure, and is thus generallyconsidered safe even for pregnant women and children.

MRI is still another imaging modality that is used to visualize thetarget tissue; MRI can be used in conjunction with ultrasound to guidethe ultrasound focus during therapy as further described below. Inbrief, MRI involves placing a patient into a homogeneous static magneticfield, thus aligning the spins of hydrogen nuclei in the tissue. Then,by applying a radio-frequency (RF) electromagnetic pulse of the rightfrequency (the “resonance frequency”), the spins may be flipped,temporarily destroying the alignment and inducing a response signal.Different tissues produce different response signals, resulting in acontrast among these tissues in MR images. Because the resonancefrequency and the frequency of the response signal depend on themagnetic field strength, the origin and frequency of the response signalcan be controlled by superposing magnetic gradient fields onto thehomogeneous field to render the field strength dependent on position. Byusing time-varying gradient fields, MRI “scans” of the tissue can beobtained.

Many MRI protocols utilize time-dependent gradients in two or threemutually perpendicular directions. The relative strengths and timing ofthe gradient fields and RF pulses are specified in a pulse sequence.Time-dependent magnetic field gradients may be exploited, in combinationwith the tissue dependence of the MRI response signal, to visualize, forexample, a brain tumor, and determine its location relative to thepatient's skull. MRI has advantages including multi-planar imagingcapability (without moving the patient), high signal-to-noise ratio,high sensitivity to subtle changes in soft tissue morphology andfunction, and no radiation exposure. But MRI suffers from itssensitivity to patient movement due to the long scan time (typicallybetween 30 minutes to a few hours), lower-resolution images of bonystructures, and interference from the operation of other radio-frequency(RF) devices.

Because each imaging technique has its own strengths and weaknesses andmay provide different types of information, it may be advantageous inpractice to combine different imaging techniques. For example, combininga CT scan with MRI can provide good details about bony structures aswell as subtle differences between soft tissues. A combination of MRIand ultrasound imaging has been shown to provide additional diagnosticinformation for better diagnosis in intraoperative neurosurgicalapplications and breast biopsy guidance. Further, because ultrasoundenergy can be employed therapeutically—e.g., to heat and ablate diseased(e.g., cancerous) tissue without causing significant damage tosurrounding healthy tissue—the combination of MRI and ultrasoundprovides imaging capability during therapeutic medical procedures. Anultrasound focusing system generally includes an acoustic transducersurface, or an array of transducer surfaces, to generate one or moreultrasound beams. In transducer arrays, the individual surfaces, or“elements,” are typically individually controllable—i.e., theirvibration phases and/or amplitudes can be set independently of oneanother—allowing the beam to be steered in a desired direction andfocused at a desired distance. The ultrasound system often also includesreceiving elements, integrated into the transducer array or provided inform of a separate detector, that help monitor ultrasound-basedtreatment. For example, the receiving elements may detect ultrasoundreflected by interfaces between the transducer and the target tissue,which may result from air bubbles on the skin that need to be removed toavoid skin burns. The receiving elements may also be used to detectcavitation in overheated tissues (i.e., the formation of cavities due tothe collapse of bubbles formed in the liquid of the tissue).

A focused ultrasound transducer system may include MR tracking coils orother markers for determining the transducer position and orientationrelative to the target tissue (such as a tumor) in the MR image. Basedon computations of the required transducer element phases andamplitudes, the transducer array is then driven so as to focusultrasound at the target. The ultrasound focus itself may be visualizedusing MRI or acoustic resonance force imaging (ARFI), and suchvisualization may be used to adjust the focus position. These methodsare generally referred to as magnetic-resonance-guided focusing ofultrasound (MRgFUS).

To successfully integrate two or more imaging systems and/or combineinformation provided by distinct systems, it is necessary to registerimage data that may be obtained in different imaging coordinate systems.Conventional approaches to registration typically involve complexcomputational procedures and may not provide sufficient accuracy fortherapeutic purposes. For this reason, utilizing such image registrationwithout verification may result in an inaccurate application of energyand unsuccessful or lengthy treatment. Accordingly, there is a need forestablishing and verifying registration of images obtained usingdifferent modalities in a sufficiently fast, reliable manner to supporttherapeutic applications.

SUMMARY

Embodiments of the present invention provide systems and methods toevaluate and quantify the accuracy of image registration obtained usingdifferent modalities and in different coordinate systems. In variousembodiments, the accuracy of the image registration relating two imagingcoordinate systems (such as a CT system and an MRI system) is evaluatedvia the use of a third imaging modality (such as an ultrasound system).For example, the spatial relationship between the ultrasound coordinatesystem and the MRI coordinate system can be determined by obtaining anMR image that includes at least a portion of the ultrasound system(e.g., some of the transducer elements) in conjunction with the knownspatial arrangement of the portion of the ultrasound system included inthe MR image. Next, the coordinates of a previously acquired CT image ofan anatomic region of interest may be transformed to coordinates in theMRI system using the image registration under study. Consequently, adistance between the anatomic region of interest and the ultrasoundsystem may be computed in the MRI coordinate system. This distance mayalso be acoustically measured using the ultrasound system, and themeasured distance in the ultrasound coordinate system may subsequentlybe transformed to the MRI coordinate system. By comparing the computedand measured distances in the MRI coordinate system, an error vector canbe assigned based on the discrepancy therebetween. This error vectorindicates the accuracy of the image registration under study—i.e., asmaller error vector indicates a smaller deviation of the computeddistance from the measured distance and thus a higher registrationaccuracy. It should be stressed, of course, that this exemplary use ofimaging modalities is for illustrative purposes only, and that any threemodalities may be used (with any of the modalities functioning in any ofthe described roles) as appropriate to the application.

In some embodiments, the present invention also provides an approach forregistering images (as opposed to verifying their registration) obtainedin the distinct coordinate systems of two imaging modalities (e.g., a CTsystem and an MRI system) using a third modality (e.g., an ultrasoundsystem). For example, the ultrasound coordinate system may be firstregistered with the MRI coordinate system via the use of an MR image ofat least a portion of the ultrasound system and the known spatialarrangement of the ultrasound system as described above. Registrationbetween the ultrasound coordinate system and CT coordinate system may beestablished via an ultrasound image and a CT image of the anatomictarget of interest. Subsequently, the MRI and CT coordinate systems maybe registered using the image registration relating the ultrasound andMRI coordinate systems and the image registration relating theultrasound and CT coordinate systems.

Although the present invention has been described with reference to theuse of the an ultrasound system for verifying and/or obtaining imageregistration between the MRI and CT coordinate systems, it is notintended that such details should be regarded as limitations upon thescope of the invention. For example, as noted above, the MRI coordinatesystem may be used to register the ultrasound and CT coordinate systems,and the CT system may be used to register the ultrasound and MRIsystems. Moreover, image registrations between other imaging coordinatesystems may also be evaluated and/or acquired by performing theapproaches described herein.

Accordingly, in one aspect, the invention pertains to a method forverifying registration of images of an internal anatomic target obtainedusing a first imaging system and a second imaging system. In variousembodiments, the method includes (a) acquiring a first image of theanatomic target and at least a portion of a third imaging system usingthe first imaging system; (b) measuring a distance between the thirdimaging system and the anatomic target using the third imaging system;(c) acquiring a second image of the anatomic target using the secondimaging system; (d) registering the first image and the second imageusing the registration; and (e) based on (i) positions of the at least aportion of the third imaging system and the anatomic target in the firstimage and (ii) the measured distance, computing an error in theregistration. In one implementation, the first, second, and thirdimaging systems include an MRI system, a CT system, and an ultrasoundtransducer system, respectively.

The method may further include obtaining the positions of the at least aportion of the third imaging system in the coordinate system of thethird imaging system; the positions may be determined based on atime-of-flight method. In addition, the method may include computing atransformation relating the coordinate system of the first imagingsystem and the coordinate system of the third imaging system based onthe positions of the at least a portion of the third imaging system inthe first image and in the coordinate system of the third imagingsystem. In one embodiment, the positions of the third imaging system aretransformed from the coordinate system of the third imaging system tothe coordinate system of the first imaging system. The second image ofthe anatomic target is transformed from the coordinate system of thesecond imaging system to the coordinate system of the first imagingsystem. Additionally, the distance between the third imaging system andthe anatomic target is computed based on the transformed positions ofthe third imaging system and transformed second image in the coordinatesystem of the first imaging system.

The distance between the third imaging system and the anatomic targetmay be measured based on signals transmitted from and received by thethird imaging system. In various embodiments, the method furtherincludes transforming the measured distance from the coordinate systemof the third imaging system to the coordinate system of the firstimaging system. The error of the registration is then determined basedon a deviation of the transformed measured distance from the computeddistance. In one implementation, the error of the registration iscompared to a predetermined threshold and the validity of theregistration is determined based on the comparison.

In another aspect, the invention relates to a system for verifyingregistration of images of an internal anatomic target obtained using afirst imaging system and a second imaging system. In some embodiments,the system includes the first imaging system for acquiring a first imageof the anatomic target and at least a portion of a third imaging system;the second imaging system for acquiring a second image of the anatomictarget; and a controller in communication with the first, second, andthird imaging systems. In one implementation, the controller isconfigured to measure a distance between the third imaging system andthe anatomic target; register the first image and the second image; andcompute an error in the registration based on (i) positions of the atleast a portion of the third imaging system and the anatomic target inthe first image and (ii) the measured distance. The first, second, andthird imaging systems may include an MRI system, a CT system, and anultrasound transducer system, respectively.

The controller may be further configured to determine the positions ofthe at least a portion of the third imaging system in the coordinatesystem of the third imaging system; the positions may be determinedbased on a time-of-flight method. In addition, the controller may beconfigured to compute a transformation relating the coordinate system ofthe first imaging system and the coordinate system of the third imagingsystem based on the positions of the at least a portion of the thirdimaging system in the first image and in the coordinate system of thethird imaging system. In one embodiment, the controller transformspositions of the third imaging system from the coordinate system of thethird imaging system to the coordinate system of the first imagingsystem. The controller transforms the second image of the anatomictarget from the coordinate system of the second imaging system to thecoordinate system of the first imaging system. Additionally, thecontroller is configured to compute the distance between the thirdimaging system and the anatomic target based on the transformedpositions of the third imaging system and transformed second image inthe coordinate system of the first imaging system.

Further, the controller may be configured to measure the distancebetween the third imaging system and the anatomic target based onsignals transmitted from and received by the third imaging system. Thecontroller may transform the measured distance from the coordinatesystem of the third imaging system to the coordinate system of the firstimaging system. In addition, the controller may determines the error ofthe registration based on a deviation of the transformed measureddistance from the computed distance, compare the error of theregistration to a predetermined threshold, and determine validity of theregistration based on the comparison.

Another aspect of the invention relates to a method for registeringimages of an internal anatomic target obtained using a first imagingsystem and a second imaging system. In various embodiments, the methodincludes(a) acquiring a first image of the anatomic target and at leasta portion of a third imaging system using the first imaging system; (b)acquiring a second image of the anatomic target using the second imagingsystem; (c) acquiring a third image of the anatomic target using thethird imaging system; (d) registering the second image and the thirdimage; (e) based on (i) positions of the at least a portion of the thirdimaging system and the anatomic target in the first image and (ii) theregistered second and third images, computing transformations relating acoordinate system of the first imaging system, a coordinate system ofthe second imaging system, and a coordinate system of the third imagingsystem. In one implementation, the first, second, and third imagingsystems include an MRI system, a CT system, and an ultrasound transducersystem, respectively.

The registration of the second image and the third image may includetransforming coordinates associated with the internal anatomic target inthe coordinate system of the second imaging system to coordinates in thecoordinate system of the third imaging system. For example, thecoordinates associated with the internal anatomic target in thecoordinate system of the second imaging system may be fitted to thecoordinates associated with the internal anatomic target in thecoordinate system of the third imaging system.

In addition, the method may include obtaining the positions of the atleast a portion of the third imaging system in the coordinate system ofthe third imaging system. In one embodiment, the positions aredetermined based on a time-of-flight method. The method may includecomputing the transformation relating the coordinate system of the firstimaging system and the coordinate system of the third imaging systembased on the positions of the at least a portion of the third imagingsystem in the first image and in the coordinate system of the thirdimaging system. In some embodiments, the transformation relating thecoordinate system of the first imaging system and the coordinate systemof the third imaging system is computed based on the transformationrelating the coordinate system of the first imaging system and thecoordinate system of the third imaging system.

In yet another aspect, the invention pertains to a system forregistering images of an internal anatomic target obtained using a firstimaging system and a second imaging system. In some embodiments, thesystem includes the first imaging system for acquiring a first image ofthe anatomic target and at least a portion of a third imaging system;the second imaging system for acquiring a second image of the anatomictarget; the third imaging system for acquiring a third image of theanatomic target using; and a controller in communication with the first,second, and third imaging systems. In one implementation, the controlleris configured to register the second image and the third image; andcompute transformations relating a coordinate system of the firstimaging system, a coordinate system of the second imaging system, and acoordinate system of the third imaging system based on (i) positions ofthe at least a portion of the third imaging system and the anatomictarget in the first image and (ii) the registered second and thirdimages. The first, second, and third imaging systems may include, forexample, an MRI system, a CT system, and an ultrasound transducersystem, respectively.

The controller may be further configured to transform coordinatesassociated with the internal anatomic target in the coordinate system ofthe second imaging system to coordinates in the coordinate system of thethird imaging system. For example, the controller may fit thecoordinates associated with the internal anatomic target in thecoordinate system of the second imaging system to the coordinatesassociated with the internal anatomic target in the coordinate system ofthe third imaging system.

In addition, the controller may determine the positions of the at leasta portion of the third imaging system in the coordinate system of thethird imaging system. In one embodiment, the positions are de based on atime-of-flight method. Further, the controller may be configured tocompute the transformation relating the coordinate system of the firstimaging system and the coordinate system of the third imaging systembased on the positions of the at least a portion of the third imagingsystem in the first image and in the coordinate system of the thirdimaging system. The controller may be configured to compute thetransformation relating the coordinate system of the first imagingsystem and the coordinate system of the third imaging system based onthe transformation relating the coordinate system of the first imagingsystem and the coordinate system of the third imaging system.

Still another aspect of the invention relates to a method for detectinga moving anatomic feature during a treatment sequence. In variousembodiments, the method includes (a) prior to the treatment sequence,(i) acquiring a first image of the anatomic feature and at least aportion of a first imaging system, and (ii) processing the first imageto compute a shortest distance between the anatomic feature and the atleast a portion of the first imaging system; and (b) during thetreatment sequence, (i) measuring the shortest distance between theanatomic feature and the at least a portion of the first imaging system,(ii) comparing the measured shortest distance to the computed shortestdistance obtained in step (a) to determine a deviation therefrom, and(iii) determining movement of the anatomic feature based on thedeviation. In one implementation, the first image is acquired using asecond imaging system; the method further includes acquiring a secondimage of the anatomic feature using a third imaging system prior to thetreatment sequence. The first, second, and third imaging systems includean ultrasound transducer system, an MRI system, and a CT system,respectively.

The method may include obtaining positions of the at least a portion ofthe first imaging system in a coordinate system of the first imagingsystem; the positions may be determined based on a time-of-flightmethod. In addition, the method may include computing a transformationrelating the coordinate system of the first imaging system and acoordinate system of the second imaging system based on the positions ofthe at least a portion of the first imaging system in the first imageand in the coordinate system of the first imaging system. In oneembodiment, the method further includes transforming the positions ofthe first imaging system from the coordinate system of the first imagingsystem to the coordinate system of the second imaging system based onthe computed transformation.

In some embodiments, the method includes registering the first image andthe second image. Additionally, the method includes transforming thesecond image of the anatomic feature from a coordinate system of thethird imaging system to the coordinate system of the second imagingsystem. The shortest distance may then be computed based on thetransformed positions of the first imaging system and transformed secondimage in the coordinate system of the second imaging system.

In various embodiments, the shortest distance between the anatomicfeature and the at least a portion of the first imaging system ismeasured based on signals transmitted from and received by the firstimaging system. In one implementation, the measured shortest distance istransformed from the coordinate system of the first imaging system tothe coordinate system of the second imaging system. Further, the methodfurther includes comparing the deviation to a predetermined thresholdand determining movement of the anatomic feature based on thecomparison.

In another aspect, the invention relates to a system for detecting amoving anatomic feature during a treatment sequence. In variousembodiments, the system includes a first imaging system for acquiring afirst image of the anatomic feature and at least a portion of a secondimaging system prior to the treatment sequence; and a controller incommunication with the first and second systems. In one implementation,the controller is configured to (a) prior to the treatment sequence,process the first image to compute a shortest distance between theanatomic feature and the at least a portion of the second imagingsystem; and (b) during the treatment sequence, (i) measure the shortestdistance between the anatomic feature and the at least a portion of thesecond imaging system, (ii) compare the measured shortest distance tothe computed shortest distance obtained in step (a) to determine adeviation therefrom, and (iii) determine movement of the anatomicfeature based on the deviation. The system may further include a thirdimaging system for acquiring a second image of the anatomic featureprior to the treatment sequence. In one embodiment, the first, second,and third imaging systems include an MRI system, an ultrasoundtransducer system, and a CT system, respectively.

The controller may be further configured to obtain positions of the atleast a portion of the second imaging system in a coordinate system ofthe second imaging system; the positions may be determined based on atime-of-flight method. In addition, the controller may compute atransformation relating the coordinate system of the second imagingsystem and a coordinate system of the first imaging system based on thepositions of the at least a portion of the second imaging system in thefirst image and in the coordinate system of the second imaging system.In one embodiment, the controller is configured to transform thepositions of the second imaging system from the coordinate system of thesecond imaging system to the coordinate system of the first imagingsystem based on the computed transformation.

In some embodiments, the controller is further configured to registerthe first image and the second image. Additionally, the may transformthe second image of the anatomic feature from a coordinate system of thethird imaging system to the coordinate system of the first imagingsystem. The controller may then compute the shortest distance based onthe transformed positions of the second imaging system and transformedsecond image in the coordinate system of the first imaging system.

In various embodiments, the controller is further configured to measurethe shortest distance based on signals transmitted from and received bythe second imaging system. In one implementation, the controllertransforms the measured shortest distance from the coordinate system ofthe second imaging system to the coordinate system of the first imagingsystem. Further, the controller compares the deviation to apredetermined threshold and determines movement of the anatomic featurebased on the comparison.

In still another aspect, the invention relates to a method for detectinga moving anatomic feature during a treatment sequence having a pluralityof treatment periods. In various embodiments, the method includesmeasuring a shortest distance between the anatomic feature and at leasta portion of an imaging system (e.g., an ultrasound transducer system)during the treatment sequence; comparing the measured shortest distancein a current treatment period to the measured shortest distance in aprevious treatment period to determine a deviation therefrom; anddetermining movement of the anatomic feature based on the deviation. Theshortest distance between the anatomic feature and the at least aportion of the imaging system may be measured based on signalstransmitted from and received by the imaging system. In addition, themethod may include comparing the deviation to a predetermined thresholdand determining movement of the anatomic feature based on thecomparison.

In another aspect, the invention pertains to a system for detecting amoving anatomic feature during a treatment sequence having a pluralityof treatment periods. In various embodiments, the system includes animaging system (e.g., an ultrasound transducer system) for measuring ashortest distance between the anatomic feature and at least a portion ofthe imaging system during the treatment sequence; and a controller incommunication with the imaging system. In one implementation, thecontroller is configured to compare the measured shortest distance in acurrent treatment period to the measured shortest distance in a previoustreatment period to determine a deviation therefrom; and determinemovement of the anatomic feature based on the deviation. The controllermay be further configured to measure the shortest distance between theanatomic feature and the at least a portion of the imaging system basedon signals transmitted from and received by the imaging system. Inaddition, the controller may compare the deviation to a predeterminedthreshold and determine movement of the anatomic feature based on thecomparison.

As used herein, the terms “approximately,” “roughly,” and“substantially” mean ±10%, and in some embodiments, ±5%. Referencethroughout this specification to “one example,” “an example,” “oneembodiment,” or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the example isincluded in at least one example of the present technology. Thus, theoccurrences of the phrases “in one example,” “in an example,” “oneembodiment,” or “an embodiment” in various places throughout thisspecification are not necessarily all referring to the same example.Furthermore, the particular features, structures, routines, steps, orcharacteristics may be combined in any suitable manner in one or moreexamples of the technology. The headings provided herein are forconvenience only and are not intended to limit or interpret the scope ormeaning of the claimed technology.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, with an emphasis instead generally being placedupon illustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A schematically depicts an exemplary MRI system in accordance withvarious embodiments of the current invention;

FIG. 1B schematically depicts an exemplary CT system in accordance withvarious embodiments of the current invention;

FIG. 1C schematically depicts an exemplary ultrasound system inaccordance with various embodiments of the current invention;

FIG. 2A depicts an approach for correlating the ultrasound coordinatesystem to MRI coordinate system in accordance with various embodimentsof the current invention;

FIG. 2B depicts an approach for correlating the CT system to MRIcoordinate system in accordance with various embodiments of the currentinvention;

FIG. 2C depicts an approach for computing a distance between eachtransducer element and the closest point on a reflective surface of thetarget in accordance with various embodiments of the current invention;

FIG. 3A depicts an approach for acoustically measuring a distancebetween each transducer element and the closest point on a reflectivesurface of the target in accordance with various embodiments of thecurrent invention;

FIG. 3B depicts the waveforms of signals transmitted from and receivedby the ultrasound system accordance with various embodiments of thecurrent invention;

FIG. 3C depicts signal processing for determining a distance betweeneach transducer element and the closest point on a reflective surface ofthe target in accordance with various embodiments of the currentinvention;

FIG. 4 depicts an approach for evaluating the accuracy of an imageregistration obtained using a conventional approach in accordance withvarious embodiments of the current invention;

FIGS. 5A and 5B depict various approaches to detecting patient'smovement during treatment in accordance with various embodiments of thecurrent invention;

FIGS. 6A and 6B depict exemplary values of the error function for twodistance measurements having the same transducer array and targetpositions at different times in accordance with various embodiments ofthe current invention;

FIGS. 7A-7D depict exemplary values of the error function for fourdistance measurements having different transducer array and targetpositions in accordance with various embodiments of the currentinvention;

FIGS. 8A-C depict various approaches to registration of two imagingcoordinate systems in accordance with various embodiments of the currentinvention.

DETAILED DESCRIPTION

FIG. 1A illustrates an exemplary MRI apparatus 102. The apparatus 102may include a cylindrical electromagnet 104, which generates therequisite static magnetic field within a bore 106 of the electromagnet104. During medical procedures, a patient is placed inside the bore 106on a movable support table 108. A region of interest 110 within thepatient (e.g., the patient's head) may be positioned within an imagingregion 112 wherein the electromagnet 104 generates a substantiallyhomogeneous field. A set of cylindrical magnetic field gradient coils113 may also be provided within the bore 106 and surrounding thepatient. The gradient coils 113 generate magnetic field gradients ofpredetermined magnitudes, at predetermined times, and in three mutuallyorthogonal directions. With the field gradients, different spatiallocations can be associated with different precession frequencies,thereby giving an MR image its spatial resolution. An RF transmittercoil 114 surrounding the imaging region 112 emits RF pulses into theimaging region 112 to cause the patient's tissues to emitmagnetic-resonance (MR) response signals. Raw MR response signals aresensed by the RF coil 114 and passed to an MR controller 116 that thencomputes an MR image, which may be displayed to the user. Alternatively,separate MR transmitter and receiver coils may be used. Images acquiredusing the MRI apparatus 102 may provide radiologists and physicians witha visual contrast between different tissues and detailed internal viewsof a patient's anatomy that cannot be visualized with conventional x-raytechnology.

The MRI controller 116 may control the pulse sequence, i.e., therelative timing and strengths of the magnetic field gradients and the RFexcitation pulses and response detection periods. The MR responsesignals are amplified, conditioned, and digitized into raw data using aconventional image-processing system, and further transformed intoarrays of image data by methods known to those of ordinary skill in theart. Based on the image data, a treatment region (e.g., a tumor) isidentified. The image-processing system may be part of the MRIcontroller 116, or may be a separate device (e.g., a general-purposecomputer containing image-processing software) in communication with theMRI controller 116. In some embodiments, one or more ultrasound systems120 or one or more sensors 122 are displaced within the bore 106 of theMRI apparatus 102 as further described below.

FIG. 1B illustrates an exemplary CT system 130 in accordance withembodiments of the present invention. The CT system 130 includes ascanner 132 having a support structure 134, an x-ray source 136, and anx-ray detector 138 on the same or opposite side of the source 136. Thescanner 132 is configured to receive a table 140 for a patient to bescanned. The table 140 can be moved through an aperture in the scanner132 to appropriately position the patient in an imaging volume or planescanned during imaging sequences. The CT system 130 further includes aradiation source controller 143, a data acquisition controller 144, anda table controller 145. The radiation source controller 143 regulatestiming for discharges of x-ray radiation from the x-ray source 136toward the patient. The data acquisition controller 144 communicateswith the x-ray detector 138 to measure the x-ray intensity datatransmitted through or reflected from the patient. The acquiredintensity data is processed by the data acquisition controller 144 or asystem controller 146 for CT image reconstruction. The table controller145 serves to adjust the table position during or between imagingsequences, depending upon the employed imaging protocol. The controllers143, 144, 145, 146 may be separate units or may be integrated as asingle unit.

In addition, the system controller 146 may be coupled to a userinterface 147 and a memory device 148. The user interface 147 may beintegrated with the system controller 146, and will generally include auser workstation for initiating imaging sequences, controlling suchsequences, and manipulating data acquired during imaging sequences. Thememory devices 148 may be local to the imaging system, or partially orcompletely remote from the system, and may be configured to receive raw,partially processed or fully processed data for CT image reconstruction.

During a typical CT scan, the x-ray scanner 132 rotates around thepatient with a predetermined speed, and the x-ray source 136 emitsnarrow beams of x-rays through the body. The x-ray intensitiestransmitted through or reflected from the patient at various angles withrespect to the patient's body are measured by the detector 138. Atwo-dimensional tomographic image (i.e., a “slice”) of the patient canbe created based on the detected beam angles and intensities of thex-rays. Multiple “slices” obtained at different angles can then beprocessed to reconstruct a CT image. In some embodiments, the CT imageof the patient is acquired at a planning stage, i.e., prior to a thermaltreatment, to allow time for the treatment plan to be prepared.

FIG. 1C illustrates an exemplary ultrasound system 150 in accordancewith some embodiments of the present invention. As shown, the ultrasoundsystem includes a plurality of ultrasound transducer elements 152, whichare arranged in an array 153 at the surface of a housing 154. The arraymay comprise a single row or a matrix of transducer elements 152. Inalternative embodiments, the transducer elements 152 may be arrangedwithout coordination, i.e., they need not be spaced regularly orarranged in a regular pattern. The array may have a curved (e.g.,spherical or parabolic) shape, as illustrated, or may include one ormore planar or otherwise shaped sections. Its dimensions may vary,depending on the application, between millimeters and tens ofcentimeters. The transducer elements 152 may be piezoelectric ceramicelements. Piezo-composite materials, or generally any materials capableof converting electrical energy to acoustic energy, may also be used. Todamp the mechanical coupling between the elements 152, they may bemounted on the housing 154 using silicone rubber or any other suitabledamping material.

The transducer elements 152 are separately controllable, i.e., they areeach capable of emitting ultrasound waves at amplitudes and/or phasesthat are independent of the amplitudes and/or phases of the othertransducers. A transducer controller 156 serves to drive the transducerelements 152. For n transducer elements, the controller 156 may containn control circuits each comprising an amplifier and a phase delaycircuit, each control circuit driving one of the transducer elements.The controller 156 may split an RF input signal, typically in the rangefrom 0.1 MHz to 4 MHz, into n channels for the n control circuits. Itmay be configured to drive the individual transducer elements 152 of thearray at the same frequency, but at different phases and differentamplitudes so that they collectively produce a focused ultrasound beam.The transducer controller 156 desirably provides computationalfunctionality, which may be implemented in software, hardware, firmware,hardwiring, or any combination thereof, to compute the required phasesand amplitudes for a desired focus location 158. In general, thecontroller 156 may include several separable apparatus, such as afrequency generator, a beamformer containing the amplifier and phasedelay circuitry, and a computer (e.g., a general-purpose computer)performing the computations and communicating the phases and amplitudesfor the individual transducer elements 152 to the beamformer. Suchsystems are readily available or can be implemented without undueexperimentation.

To perform ultrasound imaging, the controller 156 drives the transducerelements 152 to transmit acoustic signals into a region being imaged andto receive reflected signals from various structures and organs withinthe patient's body. By appropriately delaying the pulses applied to eachtransducer element 152, a focused ultrasound beam can be transmittedalong a desired scan line. Acoustic signals reflected from a given pointwithin the patient's body are received by the transducer elements 152 atdifferent times. The transducer elements can then convert the receivedacoustic signals to electrical signals which are supplied to thebeamformer. The delayed signals from each transducer element 152 aresummed by the beamformer to provide a scanner signal that is arepresentation of the reflected energy level along a given scan line.This process is repeated for multiple scan lines to provide signals forgenerating an image of the prescribed region of the patient's body.Typically, the scan pattern is a sector scan, wherein the scan linesoriginate at the center of the ultrasound transducer and are directed atdifferent angles. A linear, curvilinear or any other scan pattern canalso be utilized.

The ultrasound system 150 may be disposed within the bore 106 of the MRIapparatus 102 or placed in the vicinity of the MRI apparatus 102. Thecombined MRI-ultrasound system is known to be capable of monitoring theapplication of ultrasound for treatment and/or safety purposes. Todetermine the relative position of the ultrasound system 150 and MRIapparatus 102, the ultrasound system 150 may further include MR trackers160 associated therewith, arranged at a fixed position and orientationrelative to the system 150. The trackers 160 may, for example, beincorporated into or attached to the ultrasound system housing. If therelative positions and orientations of the MR trackers 160 andultrasound system 150 are known, MR scans that include, in the resultingimages, the MR trackers 160 implicitly reveal the location of theultrasound system 150 in MRI coordinates (in the coordinate system ofthe MRI apparatus 102). To aid in relating the ultrasound coordinatesystem to the MRI coordinate system, in some embodiments, an MR imageincluding at least a portion (e.g., some transducer elements) of theultrasound system 150 is acquired. This MR image may be used incombination with the known spatial arrangement of the ultrasoundtransducer elements to determine the locations of the transducerelements (not all of which need be included in the acquired MR image) inthe MRI coordinate system as further described below.

While penetrating the patient, ultrasound waves typically encountermultiple layers of tissues, e.g., bone, muscle, or fat, whose densityand structure, and, consequently, ultrasound propagation propertiesdiffer. Due to inhomogeneities and anisotropies in the tissues, theultrasound wave fronts are often distorted. Moreover, signals fromdifferent transducer elements may encounter different thicknesses andcontours of materials, and possibly air-filled or liquid-filled pocketsbetween transducer elements and the region to be imaged or treated,resulting in different phase shifts and attenuations. Knowledge of thestructure, density, and/or thickness of the multi-layer tissuestructures is thus important for compensating for these effects byappropriate phase shifts and amplification factors imposed on thetransducer elements, and for avoiding deterioration of focusingproperties. While MRI generally provides high-sensitivity images of softtissue (e.g., the brain), the CT scan creates images with more detailsabout bony structures (e.g., the skull). Accordingly, it may bebeneficial to combine image information obtained from the MRI apparatus102 and CT system 130. To do so, of course, requires registration of theimages and, therefore, registration of the MRI and CT coordinatesystems.

Numerous conventional approaches to image registration are available. Invarious embodiments, the accuracy of the image registration obtainedusing any desired approach is verified and evaluated using two imagingmodalities, e.g., the ultrasound system 150 in combination with the MRIapparatus 102. FIG. 2A depicts an approach 200 for relating theultrasound coordinate system 150 to the MRI coordinate system 102. In afirst step 202, spatial parameters, such as the orientations andpositions, characterizing the transducer elements 152 in the ultrasoundcoordinate system are obtained using any suitable approach. For example,each transducer element 152 may emit a pulse towards a sensor (notshown) located at the focus 158 or other position; the distance betweeneach transducer element 152 and the sensor may be determined based onthe time of flight. The actual location and/or orientation of eachtransducer element in the ultrasound coordinate system 150 can bedetermined using this determined distance. The spatial arrangement ofthe transducer elements may be stored in a memory. In a second step 204,an MR image of an anatomic target of interest (e.g., the patient's head,including the brain and skull) and at least a portion of the ultrasoundsystem (e.g., at least some elements 152 of the transducer array 153) isacquired. In a third step 206, the MRI coordinates associated with theanatomic target and/or the spatial parameters of the portion oftransducer elements 152 acquired in the MR image are determined. In afourth step 208, based on the spatial parameters of the transducerelements 152 in ultrasound coordinate system 202 (obtained in the firststep 202) and in the MRI coordinate system 204 (obtained in the secondstep 204), an image transformation matrix between the two coordinatesystems is generated. In a fifth step 210, the transformation matrix isapplied to transform the spatial parameters associated with the entiregroup of transducer elements (or at least a larger portion thereof thanthe portion acquired in the MR image) from the ultrasound coordinatesystem to MRI coordinate system. As a result of these operations, thespatial parameters associated with the transducer elements 152 arerepresented in the MRI coordinate system.

FIG. 2B depicts an approach 220 for transforming a CT imaging data setof the anatomic target from the CT coordinate system 130 to MRIcoordinate system 102. In a first step 222, the CT imaging data set isacquired during a treatment planning stage or prior to the treatmentstage. In some embodiments, the acquired CT data set is processed toextract more details about the anatomic target (step 224). For example,the CT image may include a bony structure such as a skull; the skull maybe segmented by applying a thresholding technique and/or other knownimaging processing to the acquired density values of the CT data set todetermine the surface of the skull. Thereafter, a conventional proposedapproach may be utilized to acquire an image registration between the CTand MR images of the anatomic target (step 226). Based on the obtainedMRI-CT image registration, the CT data set, including the extracteddetails of the anatomic target, may be transformed from the CTcoordinate system to the MRI coordinate system (step 228). Informationobtained using the MRI apparatus 102 and CT scanner 130 can then becombined in the MRI coordinate system to provide improved imagingquality or additional information about the anatomic target (step 230).This approach is particularly useful when the target of interestincludes both soft and bony tissues—the MRI provides high-sensitivityimages of the soft tissue (e.g., a brain) while the CT scan providesdetails about the bony structure (e.g., a skull).

Once the spatial parameters of the transducer elements and informationacquired using the CT scan are both transformed to coordinates in theMRI coordinate system, the accuracy of the MRI-CT image registration maybe evaluated. Referring to FIG. 2C, in which the anatomic target ofinterest is the patient's head, the distance 232 between each transducerelement 152 and the closest point 234 on the skull surface to thetransducer element 152 can be computed in the MRI coordinate system 102.The computed distance 232 is denoted as a registration skull distancevector (“RSDV”).

FIG. 3A illustrates acoustic measurement of this distance using atime-of-flight approach 300—i.e., based on the timing of signalstransmitted from and received by the transducer elements 152 and theknown speed of sound in the medium in which the ultrasound system isoperated (e.g., in water or air). For example, in one implementation,all (or more than 50%) of the transducer elements are set in atransmission mode to emit short acoustic pulses (˜50 μs) towards theskull (step 302). Substantially immediately after the pulse emission(e.g., a few micro-seconds after), at least some of the transducerelements are switched to a receiving mode to receive signals reflectedfrom the skull (step 304). In one embodiment, the signals received byall receiver transducer elements are recorded simultaneously (step 306).In another embodiment, the transducer elements are grouped into multiplesubsets; each subset is sequentially activated to emit and receivesignals as described above, and the received signals are sequentiallyrecorded until signals from the desired sets of transducer elements areincluded. The recorded signals can be processed to compute a distancevector (or acoustic distance vector, “ADV”) between each transducerelement 152 and the nearest point on the skull to the transducer element152 (step 308).

FIG. 3B depicts the recorded waveforms of typical ultrasound signals,where waveforms 312 represent the transmitted signals, which may be usedas references for phase measurements of the reflected signals havingwaveforms 314. The recorded signals may be processed to determine theADV. For example, also referring to FIG. 3C, the signals may be filteredat a central ultrasonic frequency (e.g., 650 KHz) with a range of 100KHz to remove noise signals from the ultrasonic signals (step 316). Anenvelope-detection approach (e.g., a Hilbert transform) may then beapplied to the filtered signals to determine an amplitude envelope 318of the reflected signals 314 (e.g., in the time window between 70 and200 μs shown in FIG. 2C) (step 320). An effective time window 322 of theamplitude envelope 318 may then be determined (step 324). For example,the starting time and stopping time of the amplitude envelope 318 may bedetermined when the signal amplitude is above a predetermined amplitudethreshold 326 and below the predetermined amplitude threshold 326,respectively. A mean waveform time of the reflected signals 314 can thenbe calculated (e.g., integrating the signals over the determinedeffective time window) (step 328). In addition, phases associated withthe reflected signals 314 in the effective time window 320 may beobtained based on the reference phases of the transmitted signals 312(step 330). Based on the determined mean waveform time and phasesassociated with the reflected signals 314, an elapsed time between thereflected signals 314 and transmitted signals 312 can be computed (step332). The ADV between each transducer element 132 and the nearestreflector to the transducer element 132 can then be determined (step334) based on the elapsed time and the speed of sound in the media. Themeasured ADV may be transformed from the ultrasound coordinate system toMRI coordinate system using the ultrasound-MRI transformation matrixobtained as shown in FIG. 2A. In this example, because the externalsurface of the skull reflects the largest fraction of the ultrasoundwaves (reflections from other interlayer skull surfaces are negligible),the nearest reflector is the closest point on the external skullsurface. Generally, the ADV measured using this approach has a highaccuracy (on the order of one-tenth of the wavelength (i.e., 0.25 mm) atthe central frequency) and can thus be suitable for evaluating the imageregistration obtained conventionally and/or detecting patient movement.

Referring to FIG. 4, the computed distance (RSDV) may be compared to theacoustically measured distance (ADV) to evaluate the accuracy of theimage registration obtained using a conventional approach. In oneembodiment, for each individual transducer element that is activated foran ADV measurement, a comparison between the RSDV and ADV is made; thecomparison may be quantified as a correlation score and/or a discrepancybetween the RSDV and the ADV using any suitable approach capable ofcomparing two vectors (such as an error function as further describedbelow). The quantified comparison values for all (or at least some)transducer elements that are activated for the ADV measurement are thenadded together to represent the “entire” correlation/discrepancy betweenthe RSDV and the ADV for all (or at least some) transducer elements.Because both the acoustic measurement of ADV and computation of RSDV inthe MRI coordinate system can be determined with high accuracy, thedeviation and/or correlation score indicates the accuracy of the MRI-CTimage registration—a smaller deviation or higher correlation scoreindicates a higher accuracy of the MRI-CT image registration.

The distance vectors, RSDV and ADV, may be compared using any suitableapproach. For example, a simple error function between the two distancevectors may be calculated as follows:

$\begin{matrix}{{{{Err}\left( {x_{shift},y_{shift},z_{shift}} \right)} = {{\sum\limits_{i = 1}^{Nel}\; \left( {X_{regi} - X_{refi} - x_{shift}} \right)^{2}} + \left( {Y_{regi} - Y_{refi} - y_{shift}} \right)^{2} + \left( {Z_{regi} - Z_{refi} - z_{shift}} \right)^{2}}},} & {{eq}.\mspace{14mu} (1)}\end{matrix}$

where X_(regi), Y_(regi), and Z_(regi) represent the values of RSDV foreach transducer element i, and X_(refi), Y_(refi), and Z_(refi)represent the values of ADV for each transducer element i; x_(shift),y_(shift), and z_(shift) represent global shift values that may bevaried to obtain a minimal value of the error function; and Nelrepresents the number of transducer elements that are activated for anADV measurement. The error function is used to evaluate the accuracy ofthe image registration. For example, if the minimal value of the errorfunction cannot be found, or if the minimal value is above apredetermined threshold, the image registration is inaccurate (or atleast not sufficiently accurate for medical treatment purposes). In someembodiments, each transducer element 152 is allowed to have an averageerror below 3 mm for registration verification purposes; accordingly, inan exemplary ultrasound system having 1000 transducer elements, thethreshold value for determining the verification of the imageregistration is set as 10⁴ mm². This value can be adjusted based on theallowable error for each transducer element. Although a distance maysometimes be miscalculated for the transducer elements (e.g., due tomeasurement noise, etc.), because the foregoing approach evaluates theimage registration by the use of a large number of transducer elements(e.g., on the order of a thousand), a small number of miscalculatedelements may not significantly affect the minimal value of the errorfunction. Accordingly, a high error value occurs only when the RSDV andADV are inconsistent or corrupted.

In various embodiments, the time evolution of ADV and/or comparisonbetween the ADV and RSDV during thermal treatment may be used to detectmovement of the patient. As used herein, the term “during treatment”connotes the overall time of a treatment session and generally includesthe time prior to, during, and after each sonication. Referring to FIG.5A, in various embodiments, each time prior to, during, and/or aftersonication, a new ADV is measured in real time. Because the new ADV maybe measured from subsets of the transducer array and/or with a lowerquality, the ADV measuring period may be short enough (e.g., 200 μs) tobe interleaved with the treatment period without interruption thereofAlternatively, one or more subsets of the transducer array 153 may bededicated to ADV measurements while the remainder of the array 153focuses ultrasound for treatment purposes. Further, a separateultrasound transducer array of elements (or, again, one or more subsetsof the elements of the transducer array 153) may be provided to receivesignals reflected from the skull for ADV measurements. The receivingtransducers, if separate, may be disposed in the vicinity of theultrasound transducer array 153, or integrated into its housing 154. Inaddition, the transducer array may be disposed within the bore 106 ofthe MRI apparatus 102 or placed in the vicinity thereof.

The newly measured ADV may then be compared to the ADV obtained at anearlier time during treatment (e.g., prior to any sonication beingperformed, a plurality of sonications before, or one sonication before).If the new ADV deviates significantly from the previously obtained ADV,the patient has likely moved in the interim. In various embodiments,when the deviation is above a threshold (e.g., 5,000 mm² for 1000transducer elements or each transducer element having an average errorof 2.2 mm, or in percentage terms, more than 5% or 10%), the patient'smovement is considered significant and corrective action may beperformed to confirm and/or compensate for the movement.

In some embodiments, the time evolution of ADV during treatment ismonitored to anticipate the patient's movement. For example, for atreatment involving 1000 transducer elements, if the ADV increases by550 mm² during each sonication, it is anticipated that the patient maymove beyond what can be clinically tolerated by the tenth sonication.Accordingly, the treatment may be suspended at the end of the ninthsonication to avoid damage to healthy non-target tissue resulting frommisalignment of the patient and the ultrasound system.

Referring to FIG. 5B, in another embodiment, movement detection isdetermined based on a comparison between the ADV and the SRDV. Forexample, after the accuracy of image registration is verified, a new ADVis measured and compared to the RSDV computed using the verified imageregistration. Again, if the difference between ADV and RSDV is below athreshold (e.g., 5,000 mm² for 1000 transducer elements), the patient'smovement may be considered negligible or within clinically tolerablelimits. If, however, the difference between the ADV and the RSDV exceedsthe threshold, the patient may have moved significantly and correctiveaction may be taken.

Accordingly, comparing the ADV to the RSDV or to a previously obtainedADV may identify a suspected patient movement without the need for imagereconstruction of the target, allowing movement to be detected in realtime. The newly measured ADV best matches the computed RSDV (or thepreviously obtained ADV)—i.e., has the smallest deviation therefrom—whenthe error function in eq. (1) has a minimal value. Accordingly, patientmovement may be detected by monitoring the value and/or the position(i.e., the values of x_(shift), y_(shift), and z_(shift)) associatedwith the minimal value. The sensitivity of movement detection may beadjusted by changing the allowed deviation threshold. FIGS. 6A and 6Bdepict exemplary values of the error function on the main axes for tworepeated ADV measurements using the same image registration (therebysame RSDV) based on the same positions of the transducer array and skullat different times. The similarity between the error values in FIGS. 6Aand 6B indicates that the approach of using the minimal value and/orposition of the error function to determine the validity of the imageregistration and/or movement of the patient is reliable and repeatable.If the patient's movement is determined based on the ADV profile (i.e.,by comparing the newly measured ADV to a previously obtained ADV), ahigher similarity of the minimal values and positions of the errorfunction are expected when there is no patient movement.

FIGS. 7A-7D depict exemplary values of the error function on the mainaxes for four ADV measurements using the same image registration butwith different skull positions. The values and positions of the minimumare shown to vary with the skull position. Accordingly and again, thecurrent invention provides a sensitive, accurate approach to verify theimage registration and/or detect patient movement.

As noted, if the comparison indicates that a significantly movement haslikely occurred, a corrective action may be performed. The correctiveaction may include acquiring a new MR image to confirm the movement,suspending the ultrasound treatment, adjusting the positions of theultrasound system and/or the patient to compensate for the movement,etc. For example, a newly acquired MR image of the anatomic target maybe checked against the last valid MR image in order to confirm themovement. This is particularly useful when only some of the transducersin the array 153 are dedicated to measuring the ADV and thereby exhibitlower sensitivity (compared with the ADV measured using the entiretransducer array). If the MR image comparison reveals that a movementoccurred, the ultrasound treatment may be suspended until the positionof the ultrasound system and/or the patient is adjusted. If, however,the MR image comparison shows that a movement either did not occur orwas not clinically significant, the treatment proceeds as planned.

In some embodiments, the combined ultrasound and MRI system providesimaging registration between the MRI and CT coordinate systems with asufficient accuracy for treatment purposes. With reference to theexemplary embodiment shown in FIGS. 8A and 8B, in a first step 802, theultrasound coordinate system is first registered to the MRI coordinatesystem using an MR image of at least a portion of the ultrasound systemin combination with information specifying the spatial arrangement ofthe transducer array in the ultrasound coordinate system as described inconnection with FIG. 2A. This ultrasound-MRI registration (which may beexpressed, for example, as a transformation matrix) allows the spatialarrangement of the transducer elements to be transformed from theultrasound coordinate system to the MRI coordinate system. In a secondstep 804, an ultrasound image of the anatomic target of interest isacquired. In a third step 806, a CT image of the anatomic target ofinterest is acquired (e.g., uploaded from a computer memory that storesimage data previously obtained during the treatment planning stage orprior to the start of treatment). In a fourth step 808, the acquiredultrasound image is compared to the CT image to register the ultrasoundand CT images. The ultrasound-CT image registration may be established,for example, by fitting the CT imaging data to the ultrasound imagingdata using correlation or any other suitable approach. In a fifth step810, a transformation (e.g., a transformation matrix) between the CT andMRI coordinate systems is computed by first transforming the CT imagingdata from the CT coordinate system to the ultrasound coordinate system(using the ultrasound-CT image registration obtained in the fourth step808), and subsequently transforming the transformed data from theultrasound coordinate system to the MRI coordinate system (using theultrasound-MR image registration obtained in the first step 802). Theobtained MRI-CT coordinate transformation thus allows images obtainedusing the two imaging systems to be combined, and thereby offers detailsabout both the soft tissue and bony structure for accurate and efficienttreatment planning.

In an alternative embodiment, with reference to FIG. 8C, the ultrasoundimage of the anatomic target acquired in the second step 804 istransformed from the ultrasound coordinate system to the MRI coordinatesystem using the transformation matrix obtained in the first step 802(step 812). The transformed image data of the anatomic target in the MRIcoordinate system is then compared to the previously obtained CT imagedata of the anatomic target in the CT coordinate system to compute anMRI-CT image registration (step 814). This can be achieved by, forexample, fitting the CT image data of the anatomic target to thetransformed image data of the anatomic target in the MRI coordinatesystem. Accordingly, the MRI-CT image registration may be convenientlycomputed as described herein.

Although the invention has been described with reference to the use ofan ultrasound system for evaluating and/or obtaining a coordinatetransformation relating the MRI and CT coordinate systems and/ordetecting the patient's movement, it is not intended for thisarrangement to limit the scope of the invention. For example, the MRIcoordinate system may be used to register the ultrasound and CTcoordinate systems, and similarly, the CT system may be used to registerthe ultrasound and MRI systems. In addition, other imaging modalitiesmay also be used in lieu of any of the above-described imagingmodalities to verify and/or obtain an imaging registration relating anytwo imaging systems using the approaches described above and/or detect apatient's movement in real time during treatment.

Moreover, it is to be understood that the features of the variousembodiments described herein are not necessarily mutually exclusive andcan exist in various combinations and permutations, even if suchcombinations or permutations are not made express herein, withoutdeparting from the spirit and scope of the invention. In fact,variations, modifications, and other implementations of what isdescribed herein will occur to those of ordinary skill in the artwithout departing from the spirit and the scope of the invention.

In general, functionality for evaluating and/or computing an imagingregistration between two or more imaging systems and/or detecting thepatient's movement as described above, whether integrated with thecontrollers of MRI, the ultrasound system, and/or the CT scanningsystem, or provided by a separate external controller, may be structuredin one or more modules implemented in hardware, software, or acombination of both. For embodiments in which the functions are providedas one or more software programs, the programs may be written in any ofa number of high level languages such as FORTRAN, PASCAL, JAVA, C, C++,C#, BASIC, various scripting languages, and/or HTML. Additionally, thesoftware can be implemented in an assembly language directed to themicroprocessor resident on a target computer; for example, the softwaremay be implemented in Intel 80×86 assembly language if it is configuredto run on an IBM PC or PC clone. The software may be embodied on anarticle of manufacture including, but not limited to, a floppy disk, ajump drive, a hard disk, an optical disk, a magnetic tape, a PROM, anEPROM, EEPROM, field-programmable gate array, or CD-ROM. Embodimentsusing hardware circuitry may be implemented using, for example, one ormore FPGA, CPLD or ASIC processors.

What is claimed is: 1-26. (canceled)
 27. A method for detecting a movinganatomic feature during a treatment sequence having a plurality oftreatment periods, the method comprising: activating an externalimage-acquisition device to measure a shortest distance between theanatomic feature and at least a portion of the externalimage-acquisition device during the treatment sequence; comparing themeasured shortest distance in a current treatment period to the measuredshortest distance in a previous treatment period to determine adeviation therefrom; and determining movement of the anatomic featurebased on the deviation.
 28. The method of claim 27, wherein the shortestdistance between the anatomic feature and the at least a portion of theexternal image-acquisition device is measured based on signalstransmitted from and received by the external image-acquisition device.29. The method of claim 28, further comprising comparing the deviationto a predetermined threshold and determining movement of the anatomicfeature based on the comparison.
 30. The method of claim 27, wherein theexternal image-acquisition device comprises an ultrasound transducersystem.
 31. A system for detecting a moving anatomic feature during atreatment sequence having a plurality of treatment periods, the systemcomprising: an external image-acquisition device for measuring ashortest distance between the anatomic feature and at least a portion ofthe external image-acquisition device during the treatment sequence; anda controller in communication with the external image-acquisitiondevice, the controller being configured to: compare the measuredshortest distance in a current treatment period to the measured shortestdistance in a previous treatment period to determine a deviationtherefrom; and determine movement of the anatomic feature based on thedeviation.
 32. The system of claim 31, wherein the controller is furtherconfigured to measure the shortest distance between the anatomic featureand the at least a portion of the external image-acquisition devicebased on signals transmitted from and received by the externalimage-acquisition device.
 33. The system of claim 32, wherein thecontroller is further configured to compare the deviation to apredetermined threshold and determine movement of the anatomic featurebased on the comparison.
 34. The system of claim 31, wherein theexternal image-acquisition device comprises an ultrasound transducersystem.
 35. The method of claim 27, wherein the shortest distancebetween the anatomic feature and the at least a portion of the externalimage-acquisition device is measured acoustically.